Constant pitch helical cone beam (CB) computed tomography (CT) is used for most applications. The pitch is defined as the ratio of the conveyor or table displacement per gantry rotation to the size of the detector array along the direction of conveyor or table movement.
For example, CT based explosive detection systems (EDS) for checked baggage screening use a constant pitch. However in carry-on baggage screening at checkpoint, the distance between the entrance of a bag and the exit of a bag is much shorter than the corresponding distance in checked baggage screening. Screeners may have to stop the conveyor belt from time to time in order to have enough time to resolve complex alarmed bags on screen.
When a line scanner is used for pre-screening followed by a CT scanner in carry-on baggage screening, it is advantageous for the CT scanner to vary the belt speed: when a bag is cleared by the line scanner, the CT scanner speeds up the belt to its maximum speed to carry the bag to the exit of the scanner without further examination; when a bag can not be cleared by the line scanner, the CT scanner returns to its normal speed to scan the bag, reconstruct CT images, and perform threat detection on the bag.
Variable pitch scanning schemes have been proposed in the prior art. Woloschek et al. (U.S. Pat. No. 6,442,228) use additional sensors to obtain dynamic parameters such as the table speed and integrate these parameters into the projection data stream for image reconstruction.
Hsieh et al. (U.S. Pat. No. 6,977,984) use a helical weighting method of reconstructing CT images with projection data acquired at variable pitch. The helical weighting method suffers significant cone beam artifacts for the CT scanners with more than eight rows of detectors. Furthermore, the method does not provide a continuous reconstruction of continuous data acquisition in the baggage screening application.